Fish finders typically use an acoustic signal that is pulsed into the water. The presence of fish in the water form obstacles to the acoustic signal. Some of the acoustic signal will reflect off the fish and return to an acoustic receiver designed to listen for the returning acoustic signal(s). It is possible to approximate the depth of the fish by recording the time taken for the return of the acoustic signal. The results can be displayed graphically such that longer return times are plotted downward from an imaginary sea level or water surface.
After a period of time, after listening for any returning acoustic signals, another pulse of the acoustic signal is repeated. The period of time for listening is repeated. Assuming that the boat has moved since the last sounding, it is possible to graphically simulate a cross section of the water under the boat. This is accomplished by displaying the results of the second listening alongside the first listening. The process of applying an acoustic signal, then listening, is repeated and the results of the listenings are sequentially plotted along side each other. The result is a plot having the appearance of a cross section of the water under the boat. Any reflections from fish will display as markings of the returned acoustic signal at various depths. The reflection off the bottom of the water column will show up as a generally continuous reflection signal often below the base of the echo signals due to fish.
The acoustic source is typically designed to project the acoustic signal mostly in a downward (or other pre-determined) direction, reducing the amount of acoustic energy going in other directions. The reason for this is that reflections of fish that are not directly under the acoustic source will produce a reflection that will be received by the acoustic receiver. Those fish will appear, incorrectly, to be under the location of the acoustic source and will also appear to be deeper than they actually are. In some instances, the fish will even appear to be below the water bottom. This is an undesirable feature, as false positioning of the fish in the water decreases the usefulness of the fish finder.
Even with efforts to directionally transmit the acoustic signal only downward, a particular sounding will “see” fish that are not yet directly under the source. Since the acoustic source is not yet directly over the fish, the reflection or echo from the fish will appear to be deeper than the actual depth of the fish. As the acoustic source moves directly over the fish, the echo from the fish will appear shallower, approaching the fish's true depth. When the acoustic source is directly over the fish, the apparent depth will match the true depth and will appear shallower than any of the deeper echoes. As the acoustic source moves away from the fish, the echoes will again appear deeper than the shallowest, true depth echo. The result is that a reflection from a fish will not appear as a single spot on the plotting, but will appear as a concave downward curve.
Similarly, any object resting on the bottom of the water column (such as rocks, weeds, etc.) or any sharp discontinuity or structure in the bottom of the water column will not be sharply imaged by current techniques. This is because the sound energy generally spreads out over a large area as it travels through the water (often described as a “cone” of energy). Thus, sound energy is reflected from objects not just directly beneath the source, but from objects in a large area around the source location. This has the practical effect of severely limiting the ability of the fish finder to sharply define bottom structure of interest to fishermen.
There is a need to improve the resolution of echoes from fish by diminishing the appearance of the concave downward curves and further resolving a fish echo into a more focused image on the plot. Similarly, there is a need to improve the resolution of echoes from bottom structure into sharp images of the actual structure of the water bottom. Sophisticated processing of acoustic signals are used in seismic imaging for petroleum exploration. These processing techniques are expensive, and designed for proper imaging of geologic formations under the surface of the water bottom. In seismic imaging, the reflected signal before the water bottom is typically muted away. The frequencies, spacings and design of the sources and receivers are typically not suited for detection of fish in the water. The myriad of processing steps to recover information from deep in the sediments and rocks remove, rather than recover, whatever remaining fish information may be in the soundings. There is a need for a different processing method that can be practically and economically applied to the particular requirements necessary to resolve fish locations, characteristics, and depths.